virtio: add ability to iterate over vqs

[ Upstream commit 24a7e4d2 ]

For cleanup it's helpful to be able to simply scan all vqs and discard
all data. Add an iterator to do that.

Cc: stable@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>